Automatic frequency control system



June 26, 1951 L. M. HOLLINGSWORTH 2,558,435

AUTOMATIC FREQUENCY CONTROL SYSTEM 2 Sheets-Sheet 1 Filed Oct. 19, 1945 ATTORNEY w 3 w H g M Mm fi i. m g D am I WW w n z. u m M. F f J. mm L w a L 1 8 & w L W n on 3206 mm. m ll|| om I h .EDUUG NNXIz IQ IW. 2 m NN WWW W W W N EN W W o. 3 T y r 9 OE J1me 1951 L. M. HOLLINGSWORTH 2,558,435

AUTOMATIC FREQUENCY CONTROL SYSTEM Filed Oct. 19, 1945 zsheets-sheet 2 H i i: l

K; 6 1 T g c N m: o D O 9 E Q U W5 |-U 5 i 1 En. I ll: II I I ...I (D gan n:j; -m

INVENTOR. LOWELL M. HOLLINGSWORTH A T TORNE' Y same as before.

stabilizing an I;-F. voltage.

Patented June 26, 1951 UNITED STATE 'IPAT am" OFFICE AUTGMATIGFR'EQUENCYCONTRODSYSTEM Lowell Hollingsworth, Cambridge, Mass., as-

.. signor, :by mesne assignments, to the United 'States of America as represented by the Secretary of War Applicatioh Octobr 19, I 945,Se1*ial No. 623388 4 Claims.

This inventionrelates in general to radioapparatus and more specifically to a system :for automatic frequency stabilization of oscillators and "similarv apparatus.

In many .types of radio object-locating devices it is necessary to control the frequency "ofalo'cal oscillators so that the intermediate frequency, resulting from mixing the output of the local oscillator with thesignal, will be constant. The

frequency of the local oscillator tends to change because of load and temperature variations and other. factors. It is sometimes desirable to change the frequency of the signal, thereby necessitating a change in frequency of the local oscillator so that the intermediate frequency will remain the The change in local oscillator frequency' must be a rapid one, as in many radio object-locating devices the signal consists of recurring pulses of extremely short duration.

When the local oscillator fproducing anoutput 1 of such 'a frequency that the resulting intermediate frequency does not have the desired value, a complete" signal pulse or chain of :pulses may be lost by attenuation unless an immediate i said oscillator.

Generally, this inventionconsists of :a discriminator connected to a trigger circuit, The output voltage of the trigger "circuit is large or.

small depending upon. which time of the trigger circuit is conducting. This voltage is used to change the frequency 'of the oscillator. "During normal operation, with control established; the

trigger circuit alternates rapidly between itstwo stable conditions. H -however, control lost,

'the output voltage will be constant; and, after a specified time, a relay circuit =operatesa thermal switch and changes the magnitude'of the output toagainrestore the oscillator to theiiproperfrequency.

Other objects, features and advantages of this invention will suggest themselves to 'thoseaskilled pin the art andwill becoine apparent fro'mg the fol- -lowin'g description of the invention'taken its-Icon- 2 nection with the accompanying drawings in which:

. Fig. 1 is a circuit diagram "illustrating :the .principles embodied in this invention; and

Fig. 2sisa chart which shows the relation between the local oscillator frequency and discriminator output, and *also illustrates the ,-possible variations in the local oscillator frequency when it is not held at a fixed frequency'by the control circuit.

Referring now more particularly to Fig. '1,

, mixer circuit l is connected to terminal to of "the primary of a transformer 12. The secondary ,POf transformer 12 is' conn'ected'across acapacitor i3 and its terminals are connected to the plates of diodes l4 and I5 respectively. Terminal -I0is also connected througha capacitor 16 to a center tap of the secondary of transformer t2 and to one side of achoke l1. The'cathodes-of'dio'des M and are connected together bythe primary of :a transformerw. Resistors [Sand 20 are connected series across the primary of transformer 1'8. Capacitors 2| and 22 are 'also'connected in series across the primary of transformer I8. The junction of resistors I9 and -20 is connected to the junction of capacitors '2 I and '22-and also to the choke I-l.

The secondary of transformer [8, the center point of which grounded, is connected between the cathodes of tubes 23 and 24. Resistors 25- and 26 are connected in series between cathodes of tubes 23 and-24. The junction of resistors 25 and-2-6 isgrounded. The plateof-tube fi is connected through a resistor 21; to a source of ,pos itive potential and also tojthe grid of tube 24 through a resistor 28 which is bypassed by a capacitor 85. The plate of tube 23 isalso' connected through a potentiometer 2!! to a negative potential. The grid of tube 23 is -connected to a negative voltagesqurce through resistor F5 and the grid of tube 24 is connected to a negative voltage source through resistor 87.

The plate of tube H'is connected turbugnresistor 30 to a positive potential source and also to the grid of tubers through resistors!- hich is "bypassed b eapacitor' 52, The plate of tube 24 is also connected'tnrough' a potentiometer 33 to-a source of negative potential. A further connection from *piateef tube: 24 is made to a potentiometer '35 through a capacitor 36. Potentioineter 35 has one; terminal grounded. The plate of 'tubeid is also connected through a potentiometer 31' and resistor 38' to a negative .potential "source. The movable contact on (potentiometer 35- is connected to the grid-ofa ture.

tube 39. The cathode of tube 39 is connected to ground through a resistor 48 and capacitor 4| in parallel. The plate of tube 39 is connected through the primary of transformer 42 to a source of positive potential. The grid of a tube 43 is connected through resistor 44 and capacitor 45 in parallel to one terminal of the secondary of transformer 42. The other terminal of the secondary of transformer 42 is grounded. The cathode of tube 43 is also grounded. The plate of tube 43 is connected to a positive potential source through the winding of a relay 46' and a resistor 41 in series.

Tube 48 is a thyratron and its plate is connected directly to the plate of tube 24. The cathode of tube 48 is grounded. The grid of tube 48 is connected through resistor 49 to the sliding contact on potentiometer 31 and the grid of tube 48 is also bypassed to ground through capacitor 50. Thejunction between potentiometer 3'! and resistor 38 is connected to terminal of relay 46.

Terminal 52 of relay 46 is connected to the terminal of resistor 38 to which negative potential 'isapplied. The normal unoperated condition of A *relay 46 is such that terminals 5| and 52 are electrically connected together, thereby shunting resistor 38.

A suitable source of electrical power (not shown) is connected between terminals 53 and 54. Terminal 53 is connected to terminal 55 Terminal 53 is also connected of thermal relay 59 and to terminal 65 of relay 46. Terminal 66 of relay 46 is connected through a resistor 61 to terminal 68 of thermal relay 59 and terminal 69 of thermal relay 62. Terminal -|0 of thermal relay 62 is connected to terminal H of relay 56. Terminal 12 of thermal relay 59 is connected to terminal 13 of relay-56. Thermal relays 59 and 62 are thermally operated relays.

Terminals 58 and 64 normally are not connected together electrically. When current passes through the heating element connected between terminals 68 and 12, heat is produced and operates' a contact to connect terminals 64 and 58 together. Similarly, terminals 6| and 63' normally are not connected together electrically. When current passes through the heating element connected between terminals 69 and 10,

heat is produced and operates a contact to connect terminals 6| and 63 together.

The movable contact of poentiometer 29 is connected to terminal I4 of relay 56. The movable contact of potentiometer 33 is connected to terminal of relay 56. The output is taken from terminal 16 of relay 56 which is alternatively connected to either of the aforesaid potentiometer contacts.

Output terminal 16 is connected to a grid 11 which controls the electron flow from the oath- .ode 18 to a strut 19 connected to the cavity resonator of an oscillator tube 88. The oscillator 86 is coupled to mixer circuit II, which combines the output-of the oscillator with a received signal to produce the I.-F. signal supplied to transformer l2. Strut 19 is heated by the current passing through it and isarranged to expand or contractwith changes in tempera- Expansion orcontraction of strut '19 local oscillator voltage.

alters the tuning of the cavity resonator and changes the frequency of the oscillator.

In operation, the I.-F. voltage whose frequency is to be controlled is applied across terminals l0 and II. This I.-F. voltage is obtained by heterodyning a radio-frequency voltage with the Diodes l4 and I5, op-

erating in conjunction with the associated apcircuit tubes 23 and 24. The trigger circuit.

action tends to make tube 24 conduct more heavily and does not change the setting of the trigger circuit. A change of polarity of the cathode trigger is required to reverse the setting of the trigger circuit. The output from the plate of tube 2310f the trigger circuit is applied to the terminal 14 of relay 56 by. means of the variable contact of potentiometer 29 and is used to control the frequency of the oscillator tube when relay 56 is in such a position that terminals 14 and 16 are connected. The output from the plate of tube 24 of the trigger circuit is applied to ter- 'minal 15 .of relay 56 by the variable contact of potentiometer 33 and is used to control the frequency of the oscillator tube when relay 56 is in such a position that terminals 15 and 16 are connected.

The output from the plate of tube 24 of the trigger circuit is also fed to tube 39 where it is amplified and applied to tube 43. Tube 39 and accompanying apparatus function as an amplifier. The output from said amplifier is applied to tube 43, which, with its accompanying apparatus, acts as a grid leak detector. When proper frequency control is established, the trigger circuit, comprising tubes 23 and 24, alternates rapidly between its two stable conditions. The resulting voltage pulses in the output thereof are amplified by tube 39 and applied to tube 43. The grid leak detection action limits the plate current in tube 43 when these pulses are present.

a If the trigger circuit remains in one stable condition for a long period, thus indicating improper frequency control, the charge on capacitor 45, produced by the previous grid leak detector action, leaks off and causes tube 43 to conduit heavily, thereby actuating the relay 46.

Relay 56, together with thermal relays 59 and 62, act as a thermal flip-flop switch. The contacts of relay 56 connected to terminals 55 and 16 are mechanically connected together and the indicated position is with terminal 16 connected to terminal 14 and terminal 55 connected to terbut tube '48, a thyratron,

and n iec imi sa, -b causi'r' g' currerit "t" flewtli ugh op coiI S-T. su'eh t flow through ewaseceirtu. action-changes the "output 'ifoltage "connection "fi'ijfil terminal to terminal 14. v v I 3 Furthermore, when relay 4B is actuated, resistor 38 isno longer shunted through terminals 5| and 51. If the trigger circuit is stable, with tube -24 conducting, when relay 46 is actuated, thegii'do'f tube '48 due the resistor and d a tor in that ci'i ciiit H wil-let conduet as the blate potentialwill be low. Iffi civeverf the g- :ger circuit is stable, with tiilie 24 notcenducting, when relay 46 is actuated, the plate potential of tube 48 will rise and in time if, when the grid reaches a particular value, thyratron 48 will fire, causing a voltage drop in resistor which is passed through resistor 3| and capacitor 32 in apparallel to the grid of tube 23 thereby causing tube 23 to be cut off and tube 24 to conduct. This action lowers the plate voltage on tube 48, causing it to be cut off.

Fig. 2 illustrates the use of the thyratron tube 48 and indicates diagrammatically the possible paths of frequency search the local oscillator will follow when not locked on the proper frequency. The local oscillator frequency is plotted horizontally and the vertical coordinate represents discriminator output voltage. Part A represents the paths of frequency search and discriminator output when the local oscillator is controlled by one tube of the trigger circuit. Part B represents the paths of frequency search and discriminator outputwhen the local oscillator is controlled by the other tube of the trigger circuit. That is, the conditions which prevail for the two positions of relay 56 are represented by parts A and B of Fig. 2. The dotted lines indicate the reversing action of relay 56 when the local oscillator is following certain paths of operation. The action of the thyratron is represented by a pulse as it efiectively reverses the searching path of the local oscillator. It will be noted that the thyratron greatly reduces the search time. t is a time slightly greater than the time required for the local oscillator to sweep from one sideband to the other. The time required to operate the thermal flip-flop switch is great enough to allow oscillator tube 80 to sweep from one end of the frequency band tothe other.

ll riseekponentially g While there has been de'sei i bed what at prese'nt considered to be the preferred emb'odi "merit 'of this invention, it will bejobvious to 'those skilled'ih theartthat va ouschanges'and ino'di'ficaitions may bemade t'hwithout =departing from theseope of the invention as set forth "the appended claims.

The invention claimed is z 1. *Inb'dihbiiaatidn, an oscillator tribe{ in'c il'uding thermal means for controllingthe oscilla t 'q'ue'nc'y, a discriminator beat frequency between "the l'a-tor rrequenc and a "sig circuit being tuned to a predetermined frequency to produce an output of onepolarityifthe bee frequency 'is above said predetermined frequerie "andfof the opposite polarity the beat frequc y is below said predetermined frequency, a {tri ger circuit including first and second 'electron ic'd is charge devices for receiving the outputfof said "discriminator circuit and arranged so that one of saidelectron'ic discharge devices conducts w en the 'output of said discriminator -cir'cuit is ol? one polarity and thehither-ofsalidelectronic-dis charge devices "conducts when the output (if said discriminator is of another polarity, means including a third electronic dischargedevice where- 'by 'Whe'nthefirst ofs'aidelectronicdischargede vices conducts current "continuously for asiifiicient interval'of time said third "electronic discharge "device conducts current heavily, thereby stopping flow of "current "in "said first electronic "discharge 'de'vice'arid starting .a iiow'of'curren t 'in'saiid second electronicdischar'ge device means whereby "the frequency of saidoscillator tube "is controlled as a function c'i the'output ofsaicl trigger circuit, means including a fourth lec} troniedisharge devicewhe'reby'when one of said first and second electronic discharge devices conducts continuously for a suificient interval of time said fourth electronic discharge device conducts current heavily, thereby reversing the effect said trigger has on the frequency of said oscillator tube.

2. An automatic frequency control system for use with a superheterodyne receiver, said system comprising means for varying the frequency of the local oscillator of said receiver, said frequency varying means being arranged to vary said oscillator frequency in a first direction in response to a signal having a first characteristic being applied thereto, and in the opposite direction in response to a signal having a second characteristic being applied thereto, a circuit for generating a signal, said circuit having two alternative modes of operation such that said signal has said first characteristic when said circuit operates in its first mode, and has said second characteristic when said circuit operates in its second mode, means for applying said signal to said frequencyvarying means, a frequency discriminator connected between the mixer of said receiver and said circuit, said frequency discriminator producing an output which is applied to said circuit for effecting a reversal in the mode of operation thereof in response to the frequency of the output of said mixer changing from one side of the intermediate frequency of said receiver to the other, said frequency discriminator being effective in controlling the operation of said circuit only over a limited range of frequencies in the vicinity of said intermediate frequency. pulse generating means coupled to said circuit, said pulse generating means being arranged to produce a pulse in response to said circuit operating in said first mode for more than a predetermined time, and means for applying said pulse to said circuit to effect a reversal in the mode of operation of said'circuit to thereby bring said mixer output within said, limited range of frequencies wherein said discriminator has control.

3. An automatic frequency control system for use with a superheterodyne receiver, said system comprising means for varying the frequency of the local oscillator of said receiver, said frequency varying means being arranged to vary said oscillator frequency in a first direction in response to a signal having a first characteristic being applied thereto, and in the opposite direction in response to a signal having a second characteristic being applied thereto, a circuit for generating first and second signals, said circuit arranged to have two alternative modes of operation such that said first signal has said first characteristic and said second signal has said second characteristic when said circuit operates in said first mode, and said first signal has said second characteristic and said second signal has said first characteristic when said circuit operates in said second mode, switching means for applying said first signal in a first position thereof and said second signal in a second position thereof to said frequency-varying means, a frequency distcriminator connected between the mixer of said receiver and said circuit, said frequency discriminator producing an output which is applied to said circuit for effecting a reversal in the mode of operation thereof in response to the frequency of the output of said mixer changing from one side of the intermediate frequency of said receiver to the other, said frequency discriminator being effective in controlling the operation of said circuit only over a limited range of frequencies in the vicinityof said intermediate frequency, normally inoperative activating; means forperiodically effecting the switching of said switching means between the first and second positions thereof and means coupled between said circuit and said activating means for rendering said activating means operative in response to said circuit operating in either one of its two modes for more than a given time, whereby the frequency of said local oscillator is scanned back and forth over a wide band of frequencies to bring the frequency of said mixer output within said limited range of frequencies wherein said discriminator has control.

4. The system defined in claim 3, further including pulse generating means coupled to said circuit, said pulse-generating means-being arranged to produce a pulse in response to said circuit operating in said first mode for more than a predetermined time, said predetermined time being less than the periodicity of said activating means, and meansfor applying said pulse to said circuit to effect a reversal in the mode of operation of said circuit to thereby reduce to a minimum the amount and magnitude of frequency hunting in restoring said mixer output to within the control of said discriminator.

LOWELL M. HOIJ..INGSWOR.TH.v

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,211,750 Humby et al 1 Aug. 20, 1940 2,267,453 Foster Dec. 23, 1941 2,434,294 Ginzton Jan. 13, 1948 

